Tires are molded in a clamshell-like arrangement under pressure and at elevated temperatures. One shell contains a male mold and the other a female, with each mold being formed of cast aluminum. Each mold has a ring shape corresponding to the shape that would result if a tire was cut along the centerline of the tread. These aluminum molds are cast from plaster blocks which are curved and tapered and fitted together to form a closed circular ring.
These blocks are called waste cores and are made from plaster which is so soft it can be crushed by hand and dissolved in water. After the casting of the aluminum mold, the waste core is flushed out leaving a clean aluminum mold which has a tire tread pattern molded therein. The aluminum molds and waste cores are made at a foundry. The waste cores which are called shells, are made from plaster blocks which have an outer rubber surface. Rubber surfaces are used because plaster casting in a plaster mold will not separate.
The pattern in the shell is opposite to that of the actual tire, that is, where the tire has rubber the shell will have air and conversely where the tire has air the shell will have material. The supplying of these rubber patterns or shells to the foundry for casting of the aluminum tire molds is a difficult and time consuming operation since it requires the accurate layout of the tire tread pattern on the curved surfaces of the plaster blocks from which the rubber coated shells are derived.
Presently, this is done patiently and very carefully by hand and eye coordination using a height gauge, measuring calipers, and a scribe. The tolerance is generally within the range of .+-.0.003 inches. Fatigue and dull instruments cause misplaced lines, lines which are too thick, and lines of varying thickness. Undetected layout errors propagate throughout the production process and ultimately appear on the manufactured tire. Tire tread designs are becoming more intricate with an increased use of curved rather than straight line segments. Patterns with mostly curved lines are difficult or impossible to lay out on a plaster block or on a rubber pattern shell. Recourse has been made to placing a drawing of the tire tread over the model block and using a pin to prick the pattern into the block.
A set of these carved blocks is the basis of a manufactured tire. The carved blocks are created from drawings supplied to a pattern shop of an independent company or from a tread layout group of the tire manufacturer. The shape of a model block is like a stone in an arch. Over the surface with the largest radius, a portion of the total tire pattern is laid out as discussed above. When the layout is completed, the pattern then is carved with delicate hand tools. Several years of the developing skills in laying out the pattern is necessary for a carver to perform effectively. When hundreds of thousands of tires are made, an excess depth in the carving process of thousands of an inch is a sensitive matter because of the accumulative cost.
To aid carvers, the surface layer is made of soft plaster for use of carving and usually is colored green to reduce eye fatigue. Underneath the soft plaster layer is a very hard plaster which is colored blue and which is difficult to carve. Tread depth is determined not by carving but by the thickness of the outer soft layer. Finally the treadware indicator level is colored yellow in the plaster mold.
A figure of revolution called a "sweep" is a multilayered ring of polystyrene, hard plaster, and soft plaster. The ring may span 330.degree.. The ring surface which corresponds to the surface of the tire to be manufactured, is smooth and is the blank sheet on which the pattern is carved. When partitioned by radial cuts into various sized segments, the model blocks for carving are obtained.
A template made of sheet metal is clamped to sturdy support bars which are clamped to a radial arm which is free to rotate about an axis. The rotating template determines the cross section of the sweep. Initially, the template has the shape for the bottom layer of the tread. Plaster which sets hard, like concrete is colored blue and is swept. After cutting a portion of the template away, the yellow tread wear indicator layer then is swept. Again, more of the template is cut away to sweep the soft green layer in which the tire tread is carved. Radial and vertical shrinkage allowance is built into the sweep to compensate for shrinkage in the cast aluminum mold.
The surface of the sweep is smooth and convex. From the top of the sweep to the bottom (from the shoulder to the centerline of a tire) the curve of the surface is called the crown. For an aluminum mold the corresponding surface is concave, and the curve is now called a cavity rather than a crown.
The model block is partitioned into segments called "pitches". The thickness of the partition is the length of the pitch. Pitches come in different lengths, but frequently they all carry exactly the same information about the tire pattern. In other words, the pattern on one pitch differs from the pattern on another pitch at most by stretching or shrinking the length or thickness of the partition. This is usually true for every pitch, and the pitch is one of the fundamental elements of a tire design. Typically, there are six pitches on a model block, nine blocks to a ring for a total of fifty-four pitches. The ordering of pitch lengths on a model block is called the pitch sequence. Similarly, the ordering of model blocks to make an aluminum mold is referred to as a block sequence. Tires with irregular ratios for pitch length roll quieter, that is, the acoustic resonance is surpressed to eliminate tire "sing".
In some designs, each pitch is quartered and the pieces are offset lengthwise. In appearance one pitch looks like a staircase while the aggregate forms four bands, each ring shaped around the tire. Other designs may have either one, two or three bands although one band is the most common. Thus, the tire pattern of a tire is defined by specifying the information in a pitch, the band width and offsets, the pitch sequence, the block sequence and the crown shapes. A block, rather than a pitch, may be the basic repetitive element in some tire tread designs. Also, a "free form" design can be accommodated.
The number of blocks that must be laid out and carved with the tire tread depends upon the number of different blocks needed to make both halves of the aluminum mold.
For example, if "A" denotes one model block and "B" denotes another block with different pitch lengths, then the sequence "AABABBAAB" describes an aluminum mold cast from nine blocks, only two of which are different. Similarly, the second aluminum mold could have a block sequence "CCDCDDCCD". This pattern would require a layout and carving of four blocks. Four recast models and/or four rubber patterns would be carved and sent to a foundry for subsequent forming of the aluminum mold.
The simplest structure to a tire tread pattern would require only carved block with pitches of equal length. At most, two blocks would be carved, the male and female halves. Through duplication, the tire would be the least expensive to manufacture. However, it would also be the noisiest due to the acoustic resonance produced due to the regular pattern. Thus, most tires are not made of such a simple design. Instead, a typical design requires two or three different model blocks to compose an aluminum mold each with either five or seven differently sized pitches.
Occasionally a tire pattern may require as many as thirteen carved blocks with up to nine different pitched lengths per block. The trend today is toward more complex designs for tire treads requiring a larger number of carved blocks and having a greater number of pitch lengths.
Thus, the need has existed for a system which will enable a tire tread pattern to be written on a complete set of plaster model blocks or on a prototype tire in a minimum time as possible and with a high degree of accuracy to eliminate the heretofore manual layout of the tire tread pattern.